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Luminescence dating of the loess profile at Dolní Vestonice, Czech Republic
Quatermtm ( ;e<;~hr
Pergamon
Printed in (ircal t'/rllalil All riThl, rcscr~cd 112~7 37tJi'tJ4 $_~{~(111
0277-3791 (94) E0032-6
LUMINESCENCE DATING OF THE LOESS PROFILE AT DOLNI VESTONICE, CZECH REPUBLIC F.M. Musson and A.G. Wintle Institute of Earth Studies, University o1' Wales. Abet3'stwyth, Dy.l~'d SY23 3DB. U.K.
Thermolumincscencc (TL) and Infra-red Stimulated luminescence (I RSL) studies were carried out on 21 samples from loess prolilcs at Dolnf Vestonice. The IRSL measurements gave svstematicalh' lo~'er equivalent doses (EDs) than the TL measurements: this could be due to inadequate preheat being applied to the former. Dose rate intercomparisons confirm the view that alpha counting should be carried out on finely ground material. Comparison of ED determination by regeneration and additive dose methods did not suggest sensitivity increases on bleaching, but for the older samples the TL ages were underestimated compared with the expected ages based on the correlation of pahleosol stratigraphy with the framework provided by the deep sea oxygen isotope record. Using the hypothesis of luminescence decay with a lifetime of 150 ka suggests that the previous ages assigned to the palaeosols nqav not bc correct.
INTRODUCTION The loess profile exposed in the brickyard at Dolnf Vestonice, a small village 35 km south of Brno in the Czech Republic. probably contains the most complete record of climatic change for the last glacial-interglacial cycle in this part of Europe. The Dolni Vestonice section was first described in 1962, and since then it has been integrated into regional syntheses of loess deposition in this area (Kukla, 1970: Kukla and Kocf, 1972). A composite section is given in Fig. 1. At the base of the main section a well-developed reddish brown soil (para-brown earth) is developed on the basal sand," loess. Above this are three dark brown humic-rich horizons (chcrnozems) separated by different thicknesses of lighter coloured loessic material. These soils, and the intervening loessic material, arc the main subject of this stud,,' as they provide evidence of periods with very different climates, which have been correlated with the deep sea oxygen isotope record (Sibrava. 1979). The lower chernozem and para-brown earth have been linked as fossil soil complex 111 (Sibrava, 1979). Together the," arc about 1 m thick and are separated by a thin loess horizon which is only a few centimctrcs thick. These two soils have been described by Kukla and Koci (1972: and references cited therein) as relating to substage 5e of the deep sea oxygen isotope record, and given the identifier B1. The chcrnozem (B l d) from soil complex III and the 1 in thick overlying loessic deposit contain a mixture of molluscs indicative of both cold and warm climates: this suggests that the," contain some rcworked material, a view conlirmed bv the description of the loessic material as "pellet sand" (Kukla. 1970). The two overlying chernozems (B2b) and (B2g) were assigned to soil complex II and have been correlated with substagcs 5a and 5c respectively (Kukla and Koci, 411
1972: Sibrava, 1979). This interpretation was partly based on the infinite radiocarbon dates obtained for humus from B2g and B2b (Vogel and Zagwijn, 1967). This would put the boundary between the B2g and the overlying loess at the stage 4/5 boundary, i.e. at 73.9 ka according to astronomical calculations (Martinson et al., 1987). Other chronological interpretations are also possible. For example, B2g and B2b could have formed during stage 3. at about 54 and 50 ka (Martinson et al., 1987). close to the limit of radiocarbon dating. Above soil complex II is about 12 m of loess which contains weaker steppe soils with lower humic contents which have been radiocarbon dated (Fig. 1). About 3 m above B2g there is a 5 cm thick pinkish soil with an associated radiocarbon date on humus of 28,300 ± 300 years BP (GrN-2(192). This soil is linked to tin archaeological occupation level found about 1 km distant from the brickvard, for which two radiocarbon dates have been obtained (29JI00 _+ 200 years BP, GrN-25t,~8: and 25,800 + 170 years BP, GrN-1286). Younger ages (15,350 + 1000 years BP, GrN-2102; and lS,40(~ + 7(10 years BP, GrN-2093) have been reported for humus from the overlying loess (Vogel and Zagxsijn, 1967). The loess deposit at Dolni Vestonicc can be compared with those in other parts of eastern Europe, e.g. in Hungary. where several metres of loess accumulated after the hist glacial maximum and before the Holocene (Wintlc and Packman, 1988). Besides the relatively hirge number of radiocarbon dates, the advantage of this site for the study of luminescence techniques is the presence of well-developed soils which have tentatively been correlated with the earlier part of the htst interghicial/ghicial cycle (cycle B of Kukla, 1970). At the very least, the soils represent significant climate changes in the past. With this in mind, the site was chosen for an extensive luminescence study, with the sampling strategy being related to the stratigraphy.
412
F.M. Mus~,or~and A.(i. ~'intlc
Uncorrected Groups TL age (ka) for Table 5
Uncalibrated radiocarbon age (years) Vogel and Zagwijn (1967)
)i,i.i.i,i.i,i.i~i.i. i,i.i.ii.i,)i.i.i.i. ~ "--~-~"- .i" " "
G"
E
15350 ± 1000 (GrN 2102)
t-"r"
)i.i.i-i,i,i,i.i,i,i.
:::::::::::::::::::::: a '::::::::::4 -.- . . ' . " . E::::=: • ' .'." :::::::::::::::::::::: b
0 Occupation layer Datum 1
18400 4- 700 (GrN 2093) 19.7±1.6 t 21.6 4- 1.9
19.6
4- 1.7 28300 4- 300 (GrN 2092) 25800 ± 170 (GrN 1286) 29000 ± 200 (GrN 2598)
[c
17.5±1.7 d 24.8±2.3
:::: :-oG::i i-i.i.i.ii.i.i-i.i.i. '.ii-i,i31.iiii
>34000 (GrN 2105)
i.i.i.i.ii.i.ii,i,i, i.i.i-i.i-71.ii.i.i. i.i.i.i.ii-i.ii.iiDatum 2
52.9 4- 4.2
.:.!.:.!.:.!.:.!.:.!:::i:::!:::!::.!.:.~...!:
ii.i.i/.i.i.i.i.i-n, i i
Soil complex I[ J
I
!iiiiiiiiiii!ii:iiiiiiiiiii?iiiiiiiiii m,:.: j
52.4 ± 4.0 ~p51.1 ± 1.4 49.7 ± 4.5 J
>51800 (GrN 2599) 42000 ± 800 (GrN 2614) 49900 ± 600 (GrN 2152)
65.0 4- 5.0 58.0 ± 6.4 62.2 ± 5.3
::::::::::::::::::::::::::::::::::::: "~ 63.7 ± 1.5 65.2 ± 5.1 J 65.8 4- 5.1 m
78.6 ± 5.4 >50000 (GrN 2604)
iiii!i:i!ilili!i!i!i!iiiiiiiii!iii!iiiii ~ 77.4 4. 6.3 ~'- 74.2 4. 3.3 t
Soil complex
III f
:!:!:!:!:!:!:!:!:!:!:i:i:!:!:i:!:!:!:! - i: 70.9 4. 5.7 9 80.0 ± 6.4 84.3 4. 6.6 "~ 84.9 ± 0.6 85.4 4. 7.4 f
:: :::::::i::::!:i!i:i:)::i::i:"; i!::i::B.! b ::iii::iii::!::!::i::!!! !!:~:i::
•i,i,i.i.i i,i,i i.i. •i i i i i i i i : ; : ; : e D ;:l S 78.3 4- 6.5 ii-i. t
LEGEND ma Luminescence sample
83,9 4- 7.1
.iii.
iiiiii=
U 84.5 + 6.5
Gamma OA spectrometer reading
iii i
i
!
~
Loessicmaterial
~
Palaeosol
FIG. 1. Schematic di;.tgf~tlll ~t lhc Doltll \ CM~HllC'L' prt~lilc ~ho~mg IJl~: Ioc;lliOll o l Jtllllill¢SCcllCu ~,anlpl¢~,, ~~allllll;t ~,pk'clfonlctcr readings and
i',ubli~h,..,.I z . ~ d , , c a i l ' , ~ , l
dak"~
l hc II
a~c~. o l ' , t a m , . : d I-,~ I h ¢ f C ~ ¢ l l C r ; i t i O l l i l l ¢ l h o d
SAMPLE i.OCATiONS T w e n t y - o n e s a m p l e s xvcre c o l l e c t e d . T h e four y o u n g est w e r e from section 1 km to the s o u t h , w h e r e a pinkish h o r i z o n was well e x p o s e d by the e x c a v a t i o n of Ioc~,s for c o n s t r u c t i o n o f the n e a r b y d a m : in T a b l e 1 the d e p t h s given r e l a t i v e to d a t u m I, the base of the section, which was 25 cm b e n e a t h the base of the o c c u p a t i o n layer. F o u r t e e n s a m p l e s w e r e t a k e n from the m a i n section in the b r i c k y a r d a n d r e l a t i v e to d a t u m 2, the t o p o f B2g. T h e t h r e e r e m a i n i n g s a m p l e s wcrc t a k e n 2 m
are
; i r e ~.ho\ ~,11.
laterally from the m a i n section w h e r e tile u n d e r l y i n g loess was e x p o s e d by the r e m o v a l of talus.
EXPERIMENTAL
DETAILS
L u m i n e s c e n c e measurements were made on the 4-11 p.m fraction. B l e a c h i n g was carried out f o r 24 hr w i t h a S O L 2 solar s i m u l a t o r and discs were preheated for 16 hr at 140°C before measurement. All discs were left for at least 24 hr b e t w e e n each
413
L u n f i n e s c c n c c Dating ol C z e c h L o e s s TABLE
1. S a m p l e , d e p t h b e l o w d a t u m a n d c q u i v a l c m d o s e ( E D ) dct,,,lrmincd by d i f l e r c n t m e t h o d s for loess I r o m D o l n i V c s t o n i c e ,
Sitmple
Depth
TL
(cm)
,
, ,r,,
,,±el
+ ,:
,re,
l,,,,
Regeneration
Additive dose
103.0-+3.b 112.2-+8.fl 93.6-+5.2 139.1-+11.6
89.(1_+4.5 96.9-+3.4 86.9±4.4 --
Rcgcncration
,,
......
TL alter IRSL
IRSL
Short shmc
Additive dose
Regeneration
Additive dose
Regeneration
Additive dose
113.6_+7.4 116_+6.7 1112.8_++1.2 14(I.2_+11.9
121.1_+4 85.11-+2.8 94.4-+6.2 1t18.9_+1.(I
84.1_+5.3 811.1_+3.6 79.4_+5.1 96.5_+7.5
IIN.8-+ 3 83.8-+2.1 95.5-+3.5 82.8-+4.8
75.3+3.6 76.7_+3.0 8(I.S+_5.6 IO5.5+_25.6
92.9-+7 78.7-+ I.fl 72.fl-+3.11 ll)S.5-+3d~
267.4+_ 14.5 231.1+_11.1 25~1.6+_ 11.2 279.4_+15.2 29(I.9-+29.8 32(~.0_+ 17.7 314.4-+8.5 342.1_+18.2 372.6_+ I l. 1 398.<)+- 111.8 -. . 412.4_+16.3 458.7+_ 1(I.7 -398.7-+19.0 . .
240.2_+20.8 217.5_+4.1 274.9_+ 18.7 323.1-+24.2 242.4-+8.2 327.4_+ 19.3 276.11+6.8 366.3+20.9 378.(I_+9.5 491.5+-8.5 -. 448.3+32.8 ---.
2118.9_+ 14.5 -2o4.9_+ 15.1 198.4-+111.1 256.2+_21.3 3311.9_+8.4 . 295.4-+14.9 . -351.11_+3.3
248.8_+5.2 -215.1 _+ 111 I81.7_+5.1 198.1-+2.7 265.9_+2(1.5
---277.5±33.5
-----
Datum 1 a b c d c
93-103 48-58 25-35 l-HI Datum 2 -211-10
f
I2-2<1
g h i i k I m n o p q r s t u
411-5(I 711-8(I 87-93 113-121! 1411-15tl 1711-178 177-183 25(1-28t 2911-31111 318-325 3311-34(l 368-373 445-455 5(}5-515 555-565
269.6_+8.9 257.3_+3.4 254.0_+ 19.0 2811.1_+7.8 253.(1+28.3 434.8_+25.8 314.4-+8.5 351.0+25.4 --369.6_+8.2 434.5_+15.7 412.9-+11.2 427.7_+25.7 316.9_+ 19.(I 398.3-+23.1 377.5-+ 1(I.8
TL Thermotuminesccnce.
186.8_+4. I 2511.7-+7.11 249.9_+7.5 229.11-+4.6 182.8-+9.9 398 all-+ 111.8 3111.1_+15.2 373.4-+11.5 --377.1_+32.0 . 451.7_+20.1 397.9_+ 14.9 265.(I_+5.22 -.
224.5-+211. I 259.8+_6.6 195.1_+7.2 2(111.1_+5.9 227.3_+6.5 294.3_+ 11.9 253.0_+2(I.I 317.2-+25.(I 242.11+_2(I.7 21111.8-+13.3 2t111.7_+22.S 357.7_+ 27.b . . . 330.1_+211.9 310.7-+11.9 . . . 353.0_+S.4 426.11-+5.9 349.0+_28.2 512.1-+65.3 . . . 359.2-+4 412.5-+19.3 431. I _+ 15.3 -3 5 5 . 8 _+ 13.3 -363.9_+2,'.;.3 -. . .
324.9-+~.t~ -399.1+_8.6
I R S L Infra-red stimulated l u m i n e s c e n c c .
operation. Measurements were carried out in an automated Rista TL/OSL Reader (Botter-Jensen et al., 19911 with a Schott BG-39 filter being used for the infra-red stimulated luminescence (IRSL) and a Coming 7-59 and Chance Pilkington HA-3 combination for the thermoluminescence (TL). The integrated luminescence for 100 seconds IR exposure was used for the IRSL equivalent dose (ED) determination, and the same discs then had their TL measured. Thermoluminescence measurements were also made on a separate set of discs. The ED was also determined
by a 0.1 second IRSL measurement taken before the TL measurement (short shine data in Table 1). Both the regeneration and additive dose methods of ED determination were applied and the results are given in Table 1. Dose rates for all samples were obtained from the potassium content, as measured by atomic absorption spectrometry, and alpha counting of bulk samples which had been ground in a stainless-steel ball mill. The K:O content and ground alpha counts are given in Table 2, along with the total dose rate calculated
T A B L E 2. Radioactivity analyses, equivalent d o s e s ( E D s ) . total d m c r a t e s a n d a g e s c , d c u l a l c d l r o m t h e m Sample
Depth below datum
Alpha count
K,O content
ED
Total dose rate
(cm)
(ground)
("4,)
(G})
linG,, a ~)
0.95 0.93 11.96 1.03
,." 16 2.1~ _._0~ "~ 2.24
lU3.u_+3.<~ 112.2-+ s.
5.24_+0.13 5.19-+0.14 5.32_+11.15 5.,'Q_+I). 15
19.71111_+ 16<111 21 .NM)_+ 19(11) 17.5(1(I_+ 17(111 24..sa )fl-+ 231111
0.90 O.85 0.89 11.73 I).74 0.94 11.83 0.96 11.78 (I.89 0.911 fl.95 11.85 fl.85 (I.78 (I.78 I).64
.- _
2(lt).6+_S.9
2.211 _"~ . _ 8"~ ' 2.1)fl 2.1111 2.211 _.,fl'~ ~ _._11 "~ "~ 2.3O 2.311 2.36 2.411 2.18 '3 "3 _.3_ 2.18 2.36 2.68
257.3_+34 254.11+ 19.11 28 I. I -+7 S 253.11+_38.3 3 2 6 . f l + 17.7 314.4-+N.5 351 . I+~q __ 4 3 7 2 . 6 ± I I. 1 39.'-;.11_ +_ 1{I.8 369,6_+ 8.2 4 3 4 . 5 _+ 15.7 4 1 ~_ . ~) _+ 11._ 4 _3 7 . 7 _- 4_- 3 ~- . 7 361.t1±19.11 398.3_+23. I 3 7 7 . 5 +_ Iq).8
5. Ill-+ft. 14 4.t) l _+0.13 5.11 ±ft. 14 4.31 -+I). 12 4.3(i-+11.12 5.24-+11.14 4.82+o_. 13 5.33_+11.15 4.74_+11.13 5.14_+11.14 5.21 ±ft. 14 5.43_+0.15 4,9()+(I.13 . q.l}l _+(I. 13 4.6__0.~+ 1_'~ 4.75_+O. 1 I 4.47-+O. 12
52.9(11)_+ 42(111 52,4111)_+4111111 49.7(M)_+ 45(111 fi5 ,(1(11I-+ 5(11111 58.(11111_+64(111 62,2(111-+53(111 /+5,21R)-+5 I<111 fi5,bl[111-+51111) 78.6(111-+64(111 77.41R)+ 631111 711.9(11)_+571R)
Age ()'cars)
( c t / u n t s ks I c m 2)
Datum 1 a b c d e f g
h i j k
1 m n o p q
r s t u
93-1113 48-58 25-35 l-Ifl Datum 2 -2(1-111 2-2fl 411-511 71>811 87-93 113-1211 14(1-15fl 171#-178 177-183 251#-2611 291t-311(I 318-325 330-340 368-373 44q~tSq. .. 5(15-515 555-565
£0,(1111)-+ 64(R) 84.300+_ (~(lll 85,400-+ 74(•) 78,3(10-+65011 83,91111-+7 I1111 84,5(111_+1"15(111
F.M. M u s s o n and A . G . Wimlc
414
"IAI?,LE 3. A l p h a count rates and dr',', inlinitc beta (lose rates calculated lrom thick ~,ourcc alpha (TSA(') and beta (TSBC) counting Sanlplc
D e p t h bclm~ dattun (cm)
Alpha count crushed
A l p h a count ground
(countslkscc/cm 2)
(counls/kscc/cm 2)
A l p h a count ratio (crush/groundl
(I.92 1.24 I. 12 1.24
I).95 11.93 ILt)6 1.03
11.97 . ~, I.'~ I. 17 1,20
I.()S 1. I3 t).93 0.S6 ().S4 I.(ll I. 16 0.gN
O.tlU ().8g 0.89 11.73 0.74 11.t)4 (I.83 (I.t)(~ 7,',;
1.20 1 ~'~ 1.0S I . IS 1.14 I.(17 1.411 I.t12
I)atum I 93-103 4S-5S 25-35 1 Ill I)al tllll 2 - 2 0 - 111 I--40-51t 71l-gll S7 93 113-1211 1411 150 I70-17S 177- IS3
a b
c d c 1 ,, h i i k 1 m II
_
o
.
- -
.
r s t u
--
(I.,,49
I).92 1.18
0.90 0.95
(G_6(I
290-300 3IX 325
p
.
3aS-373 44~,-4~ . ~ 5(15-515 555-565
If.
.
.
I. IIt) {I.g6 (I.89 1.113
.
.
11.85 0.78 . ().7s I).h4
"I'ABI.E 4. C o m p a r i s o n ol g a m m a a n d beta (lose late', { i n ( i \ a i) c a l c u l a t e d J'lOlll different lllCiiMilClllClll,,
A B C D IZ F G
A
1.34_+11.(~4 1.37+OUl 1.37_+(L03 1.25+_0.02 1.411_+1L113 1.35+11.(13 1.23_+o.03 '
B:
(::
I )~
.40=cuC
2.25+_H{pu 2.17+{L12
42+_o.o2
2.37_+u.1~4 2.41 _+(I.It4 2.13_*.1UI4 2.52+1L1~4 2.33+11.04
.51 *_11.1)2 .411+_1L112 .3II_+(LII2 .222[UI2 .411+_0.02
2.211"_1L(14
2.27+(i.1ff~ 2.3N_+11.05 2.4S_+0. Ill 2.5h~0.11 2.~7~11 tl4 2.4114-_11.115
hi 3#11 gallllll~lusing gi.lllllllilspccIrOlllCt,2r CldlllCl1[ COlllpOMtion.
-, I) D g a m m a c a h : u l a l c d using g r o u n d a l p h a count and K,(). :~: Beta dose rates using galllllla spectrometer ¢lclllCnI composition tlS,Jllg ill Sitl¢ water contellt, Bctll dr)so raD2s fronl dr~ beta couiitiil~.
Beta dose rate TSB(" ( m G 3 a I)
_._9( "~ "~ _..~81 "~. - ' 2.583 2.635
2.318 " "~114 2.437 2.448
2.33-+O. lO 2.35_+0.1H 2.51 _+IL04 2,43_+t1.05
2.3114
2.41 +0.O4 2.40_+0.02 _.4)_IL0_'~ -+ "~ 2. I l +11.[11 2.10-+IL06 2.34_+ 0.1if-, 2.4()_+1).II2 2.40_+11.03
.
1.2S 1.. Ill 1.14 1.60
.
3 _.3_9 2.510 "~ "~S'~ 2. 134 2.114 2.4O3 2.53(~ _.37 . .
- -
assuming a water content of 20% based on natural water contents measured on Czech loess (Feda. 19661 and laboratory measurements. The measured in situ water content at 30 cm from the surface was only 5% as the site was visited in the summer. Alpha counting of samples crushed with a pestle and mortar (Table 3) gave counts which were consistently higher than the ground alpha counts (from 1.05 to 1.60 times) with one exception (sample a). These results contirm the study of Z611er and Pernicka (1989), who found cvidence of over-counting for coarser material. Use of the ground sample alpha counts in the dose rate calculations gave ages in better stratigraphic ordcr. Thick source beta counting (Sanderson. 1988) was also employed using 15 g crushed samples, which ~vcre counted fl~r three hours in rotation with MgO (background)and a standard composed of powdcrcd Shap Granitc (with a dry beta dose rate o f 6.25 m G v a t). T h c dry bcta d o s e rates obtained in this wa~ arc given in Table 3. It was not possible to m a k c g a m n l a s p e c t r o m e t e r readings down the whole section. However. ,(t s e v e n
Sample
Beta dose rate ground TSA(" I m O y a ~)
.
_.4.~ " "I 2.691
1.02 1.24
.
.
.
Beta dose rate crushed TSA(" ( m G y a ~)
2.349 2.014 .~.0_. " '", 2.33~ 2,336 .
.
. _._70 2.36(t 2.411 2.484 4~
2.73_+11.117 _.44_0.06 _. 14+0.04
_.:,44 "~ "
2.66_+(I.I15
_." 179 2.3113 2.40()
~ ~(1+_(1.1)7 2.17_+( I14 2.32_+0.115
.
2.55 ~ . ."~. . "~.~t) . 2.409 2.748
- -
locations (shown in Fig. 1) the gamma dose rate was recorded in situ (Table 4). For samples from these locations the gamma dose rate was also calculated from the ground alpha count and K~O analysis. Also in Table 4, the values of the beta dose rates calculated from the four-channel gamma spectrometer readings are compared with those from thick source beta counting (TSBC). The in situ water content was < 5 %.
DISCUSSION Equivalent doses were obtained by both the regeneration and additive dose TL methods (by integrating the 25(I--401)°C region of the glow' curve) on discs which had previously had their IRSL signal measured for I()0 seconds. The E D s for the regeneration method w e r e on average 15% higher, suggesting that no sensitivity increase has resulted from the laboratory bleach. For ,wmn"er~ samples, which give TL age estimates of 6() ka or less, there appears to have been a sensitivity decrease as a result of laboratory bleaching. For the set of discs on which TL alone was measured, only I. o a n d q show higher E D s for the additive dose method. Although these three are towards the base of the section, this effect does not appear to bc systematic, as samples r and s show the inverse. For the second set of TL measurements (those after the 1()0 sec IR exposure), the EI)s for lhc additive dose method tended to be higher for the older samples, with i and k being exceptions. The values of ED by the regeneration method for TL are shown in Fig. 2. The E D s for samples s. t and u arc lower than for the overlying samples as a result of their lower total dose rates (Table 2). These are the values used to obtain the
415
l.ummescencc Dating of Czech Loess o 2 ,t a.
[;
-C
t
-2 1o
•
A&
1:I1
I)
1O0
:20o
300
-I00
500
El) ((;v FIG.
_"
Equiv,dcnt
dose vs. depth curve measurements.
for
regeneration
ages in Table 2. These EDs were selected as there was a more complete data set and the results were much less scattered on a depth-dose plot (Fig. 2). (Note that the EDs are from the first set of T L measurements, apart from those for samples m. n and j.) For the IRSL measurements the regeneration/additive dose ratio is 0.91 for the 100 sec IR exposure and 1.02 for the short shine. However, the IRSL EDs arc smaller than those for the TL; for the regeneration data sets obtained on the same discs, the mean IRSL underestimate was 15%. This may be due to the incomplete removal of the unstable part of the IRSL signal by the preheat. The 16 hr preheat was selected following storage experiments at 140°C. After 10 hr at this temperature the IRSL signal from natural and irradiated aliquots attained a fixed ratio. The short shine regeneration EDs are slightly lower still, being on average 5 % lower than those for the signal integrated over the first 1{)(Isec. This may indicate a loss of natural signal as a result of laboratory illumination during the disc preparation. For the following discussion the ages are those given in Table 2 based on the ED obtained using TL regeneration measurements. Five groups can be recogniscd which are related to the visible soil horizons and the loess overlying the pinkish layer. The mean T L ages arc ,aiven in Table 4 and may be compared with other age evidence. The TL ages relating to the loess overlying the pinkish laver (a, b and c) are in agrcemcnt with the radiocarbon ages. The loess beneath the pinkish layer (d) appears to be younger than expected on the basis of the radiocarbon dates for the occupation laver at the excavation site, and the quoted uncalibrated radiocarbon dates are already about 2 ka less than the calendar age (Bard et al., 1990). The most likch explanation for such a discrepancy is that the pinkish horizon at the sampling site is rcworkcd matcrial derived from a nearby occupation horizon, althou,,he there is no sedimentological evidence to support this hypothesis. No such age discrepancy was encountered when dating loess at Stillfried, Austria, from a horizon with a similar radiocarbon age (Wintle, 1987). The ages of the older groups are systematically
younger than might be expected according to the interpretations of Kukla and Koci (1972) and Sibrava (1979). The chernozems B2g and B2b do not fall within the accepted age range of stage 5, 129.7-73.9 ka (Martinson et al.. 1987) and soil complex Ill ( B i b and Bid) does not fall within the range for substage 5e, 129.7-122.5 ka. Using the same TL dating procedures, loess bracketing the substage 5e soil in Hungary gave ages of about 76 ka (Wintle and Packman, 1988) and in Yugoslavia ages of 7 5 - 8 5 ka (Singhvi et al., 1989). Similar age limitation has been reported elsewhere in Europe (see Wintle, 1990, for further discussion), but not in New Zealand (Berger et al., 1992). A possible cause of underestimation, particularly if there is no obvious sensitivity change (as evidenced by the lack of systematic underestimation of the regeneration ED compared with that obtained by the additive dose method) is the long-term loss of luminescence centres (Debenham, 1985). If this loss can be characterised by a formula of the form T = "r(l-e '/') where -r is the lifetime of the decay and t is the true geological age, then the luminescence age T can be predicted if a suitable value of "r is proposed. "r is the limiting age which would be obtained for an infinitely old sample which behaves in this manner, and a value close to 150 ka has been suggested (Wintle, 1990). Taking the Oxygen Isotope Stage 4/3 boundary as 59 ka, and the end of the Oxygen Isotope Substages 5a, 5c and 5e as 73.9, 96.2 and 122.5 ka, respectively (Martinson et al., 1987), the corresponding luminescence ages calculated from the above formula would be: 4/3 boundary, 7" = 48.8 ka: 5a/4 T = 58.4 ka; 5c/5b T = 71.0 ka: and 5e/5d T = 83.7 ka. Assuming that the average luminescence ages of a soil will be biased toward the end of the warm phase giving rise to it, the TL ages for groups 2, 3, 4 and 5 in Table 5 can be compared with the ages calculated above. (The assumption is based on the role of bioturbation in bringing mineral grains to the surface where they arc exposed to light.) It should be pointed out that this model only takes into account a simple time-dependent behaviour. This may need to be modified, ahmg the lines suggested by Mejdahl (1988), to incorporate the additional effect of variable dose rates from site to site. This is not easily demonstrated for European loess, for which most dose
T A B L E 5. Mean lhcrmoluminesccnec (TL) ages for lhc main straligraphic units at the Dolni Veslonice section Group 1 2 3 4 5
Unils
M e a n T L a g e (ka)
Samples
Pinkish laver and loess above Upper chcrnozem B2g Lower chernozem B2h ('hernozcm Bid Para-brown earth B I b
19.6+ 1.7
a,b,e
51.1_+1.4 63.7+ 1.5 74.2_+3.3 84.9+0.6
f,g i,k n.o q.r
416
F.M. Musson and A.G. Wintlc
rates arc of the order of 4 to 6 m G y a -~. as at Dolni Vestonice. Ages well over the 150 ka limit proposed bv D e b e n h a m (1985) have been obtained in New Zealand (Berger et al., 1992), where much lower dose rates were measured. Occasionally, older ages are also found in Europe where the dose rates are lower. In spite of this limitation, the effect of D e b e n h a m ' s model at this site (where dose rates range from 4.31 to 5.62 m G y a -~) was explored in terms of the ages proposed by others (Kukla and Kocf, 1972; Sibrava, 1979) for the two soil complexes II and II1. Comparison of the effective TL ages calculated for the oxygen isotope boundaries (given above), assuming the model with the mean TL ages obtained for the observable soil units (Table 5), suggests that the soils are in any case wrongly grouped. The para-brown earth ( B l b ) is of a different age from the overlying chernozem (Bld). According to the model proposed in this paper, the para-brown earth ( B i b ) would correspond to substage 5e, the overlying chernozem (Bld) to substage 5c, and the lower chernozem (B2b) of soil complex lI to substage 5a. This suggests that a much higher rate of loess deposition occurred in this region during substage 5b than 5d. The upper chernozem (B2g) would have formed significantly later, although it seems too old to relate to the warm phase at the beginning of stage 3 (event 3.31 of Martinson et al., 1987), which has an astronomical age of 55 ka, and thus would give a calculated T L age of 43 ka. Instead, a warm phase in stage 4 ending around 63 ka would need to be invoked to result in the T L age of 51 ka obtained for group 2. However, it should be noted that the age of the loess 15 cm above the top of B2g (sample e) has a similar age; B2g could therefore represent reworked humic material from an earlier chernozem, e.g. B2b. Studies of soil micromorphology are required to investigate this hypothesis.
CONCLUSIONS The TL age estimates obtained for samples from the four soils (para-brown earth and three chernozems), which are superimposed at Dolnf Vestonice, suggest that they formed at different times within the last interglacial-glacial cycle, in contradiction to previous interpretations based on sedimentological analyses. H o w e v e r , the age estimates from the TL and IRSL m e a s u r e m e n t s using regeneration procedures underestimate the ages, as has been found for other sites in central Europe. The interpretation explored in this paper, based on D e b e n h a m ' s hypothesis of luminescence centre decay with a lifetime of 15{) ka, is that the para-brown earth, and the two overlying chernozems at Dolnf Vestonice, relate to Oxygen Isotope Substages 5e, 5c and 5a, respectively. Comparison of the E D s from TL and I R S L m e a s u r e m e n t s on the same discs suggest that a 16 hr preheat at 140°C is not adequate to isolate a stable I R S L signal. The behaviour giving rise to
the age underestimation in T L also affects the IRSL measurements.
ACKNOWLEDGEMENTS The authors thank Dr M.L. Clarke for her critical comments and assistance with the preparation of the manuscript, and H. Edwards for determination of the K~O contents. AGW thanks Dr R. Grtin and Dr P. Havlicek of the Czech Geological Survey for assistance in the field and Professor H.P. Schwarcz for the loan of the gamma spectrometer. Financial assistance was provided by NERC grant GR3/7242. This is publication number 349 of the Institute of Earth Studies, University of Wales, Aberystwyth.
REFERENCES Bard, E., Hamelin, B., Fairbanks, R.G. and Zindler, A. (1990). Calibration of the lzC timescale over the last 30,000 years using mass spectrometric U-Th ages from Barbados corals. Nature, 345, 405-41(I. Berger, G.W., Pillans, B.J. and Palmer, A.S. (1992). Dating loess up to 800 ka by thermoluminescence. Geology, 20, 403-406. Botter-Jensen, L., Ditlefsen, C. and Mejdahl, V. (19911. Combined OSL (infrared) and TL studies of feldspars. Nuclear Tracks and Radiation Measurements, 18, 379-384. Debenham, N.C. (1985). Use of U.V. emissions in TL dating of sediments. Nuclear Tracks and Radiation Measurements, 10, 717-724. Feda, J. (1966). Structural stability of subsident loess soil from Praha-Dejvice. Engineering Geology, !, 2[11-219. Kukla, J. (1970). Correlation between Ioesses and deep sea sediments. Geologiska F6reningen i Stockhohn FOrhandingar, 92, 148-1811. Kukla, G.J. and Koci, A. (1972). End of the last interglacial in the loess record. Quaterna O' Research. 2, 374-383. Mejdahl, V. (1988). Long term stability of the TL signal in alkali feldspars. Quaterna O' Science Reviews, 7, 357-36[). Martinson, D.G., Pisias, N.G.. Hays, J.D., Imbrie, J., Moore, T.C., Jr and Shackleton, N.J. (19871. Age dating and the orbital theory of the Ice Ages: development of a high-resolution 0 to 300,00[~-year chronostratigraphy. Quaternary Research. 27, 1-29. Sanderson, D.C.W. (1988). Thick source beta counting (TSBC): A rapid method for measuring beta doserates. Nuclear Tracks and Radiation Measurements, 14, 2113-2(/7. Sibrava, V. (1979) ht: Guide to Excursions ~1 the Sixth Session of the IGCP Project 24, pp. 65-6,6. Geological Survey, Prague. Singhvi, A.K., Brongcr, A.. Saucr, W. and Pant, R.K. (1989). Thermoluminescence dating of Ioess-palaeosol sequences in the Carpathian Basin (cast-central Europe): A suggestion for a revised chronology. (Jwmical Geology, 73, 307-317. Vogel, J.C. and Zagwijn. W.H. (1967). Groningen radiocarbon dates, VI. Radiocarbon. 9, 63-1116. Wintle, A.G. (1987). Thermoluminescence dating of loess at Rocourt, Belgium. Geologic en Minjhouw, 66, 35-42. Wintle, A.G. (1990). A review of current research on TL dating of loess. Quaternary' Science Reviews, 9, 385-398. Wintle, A.G. and Packman, S.C. (19881. Thermoluminescence ages for three sections in Hungary. Quaternary Science Reviews, 7, 315-320. Z611er, L. and Pernieka, E. (19891. A note on overcounting in alpha counters and its elimination. Ancient TL, 7, 11-13.
Pergamon
Printed in (ircal t'/rllalil All riThl, rcscr~cd 112~7 37tJi'tJ4 $_~{~(111
0277-3791 (94) E0032-6
LUMINESCENCE DATING OF THE LOESS PROFILE AT DOLNI VESTONICE, CZECH REPUBLIC F.M. Musson and A.G. Wintle Institute of Earth Studies, University o1' Wales. Abet3'stwyth, Dy.l~'d SY23 3DB. U.K.
Thermolumincscencc (TL) and Infra-red Stimulated luminescence (I RSL) studies were carried out on 21 samples from loess prolilcs at Dolnf Vestonice. The IRSL measurements gave svstematicalh' lo~'er equivalent doses (EDs) than the TL measurements: this could be due to inadequate preheat being applied to the former. Dose rate intercomparisons confirm the view that alpha counting should be carried out on finely ground material. Comparison of ED determination by regeneration and additive dose methods did not suggest sensitivity increases on bleaching, but for the older samples the TL ages were underestimated compared with the expected ages based on the correlation of pahleosol stratigraphy with the framework provided by the deep sea oxygen isotope record. Using the hypothesis of luminescence decay with a lifetime of 150 ka suggests that the previous ages assigned to the palaeosols nqav not bc correct.
INTRODUCTION The loess profile exposed in the brickyard at Dolnf Vestonice, a small village 35 km south of Brno in the Czech Republic. probably contains the most complete record of climatic change for the last glacial-interglacial cycle in this part of Europe. The Dolni Vestonice section was first described in 1962, and since then it has been integrated into regional syntheses of loess deposition in this area (Kukla, 1970: Kukla and Kocf, 1972). A composite section is given in Fig. 1. At the base of the main section a well-developed reddish brown soil (para-brown earth) is developed on the basal sand," loess. Above this are three dark brown humic-rich horizons (chcrnozems) separated by different thicknesses of lighter coloured loessic material. These soils, and the intervening loessic material, arc the main subject of this stud,,' as they provide evidence of periods with very different climates, which have been correlated with the deep sea oxygen isotope record (Sibrava. 1979). The lower chernozem and para-brown earth have been linked as fossil soil complex 111 (Sibrava, 1979). Together the," arc about 1 m thick and are separated by a thin loess horizon which is only a few centimctrcs thick. These two soils have been described by Kukla and Koci (1972: and references cited therein) as relating to substage 5e of the deep sea oxygen isotope record, and given the identifier B1. The chcrnozem (B l d) from soil complex III and the 1 in thick overlying loessic deposit contain a mixture of molluscs indicative of both cold and warm climates: this suggests that the," contain some rcworked material, a view conlirmed bv the description of the loessic material as "pellet sand" (Kukla. 1970). The two overlying chernozems (B2b) and (B2g) were assigned to soil complex II and have been correlated with substagcs 5a and 5c respectively (Kukla and Koci, 411
1972: Sibrava, 1979). This interpretation was partly based on the infinite radiocarbon dates obtained for humus from B2g and B2b (Vogel and Zagwijn, 1967). This would put the boundary between the B2g and the overlying loess at the stage 4/5 boundary, i.e. at 73.9 ka according to astronomical calculations (Martinson et al., 1987). Other chronological interpretations are also possible. For example, B2g and B2b could have formed during stage 3. at about 54 and 50 ka (Martinson et al., 1987). close to the limit of radiocarbon dating. Above soil complex II is about 12 m of loess which contains weaker steppe soils with lower humic contents which have been radiocarbon dated (Fig. 1). About 3 m above B2g there is a 5 cm thick pinkish soil with an associated radiocarbon date on humus of 28,300 ± 300 years BP (GrN-2(192). This soil is linked to tin archaeological occupation level found about 1 km distant from the brickvard, for which two radiocarbon dates have been obtained (29JI00 _+ 200 years BP, GrN-25t,~8: and 25,800 + 170 years BP, GrN-1286). Younger ages (15,350 + 1000 years BP, GrN-2102; and lS,40(~ + 7(10 years BP, GrN-2093) have been reported for humus from the overlying loess (Vogel and Zagxsijn, 1967). The loess deposit at Dolni Vestonicc can be compared with those in other parts of eastern Europe, e.g. in Hungary. where several metres of loess accumulated after the hist glacial maximum and before the Holocene (Wintlc and Packman, 1988). Besides the relatively hirge number of radiocarbon dates, the advantage of this site for the study of luminescence techniques is the presence of well-developed soils which have tentatively been correlated with the earlier part of the htst interghicial/ghicial cycle (cycle B of Kukla, 1970). At the very least, the soils represent significant climate changes in the past. With this in mind, the site was chosen for an extensive luminescence study, with the sampling strategy being related to the stratigraphy.
412
F.M. Mus~,or~and A.(i. ~'intlc
Uncorrected Groups TL age (ka) for Table 5
Uncalibrated radiocarbon age (years) Vogel and Zagwijn (1967)
)i,i.i.i,i.i,i.i~i.i. i,i.i.ii.i,)i.i.i.i. ~ "--~-~"- .i" " "
G"
E
15350 ± 1000 (GrN 2102)
t-"r"
)i.i.i-i,i,i,i.i,i,i.
:::::::::::::::::::::: a '::::::::::4 -.- . . ' . " . E::::=: • ' .'." :::::::::::::::::::::: b
0 Occupation layer Datum 1
18400 4- 700 (GrN 2093) 19.7±1.6 t 21.6 4- 1.9
19.6
4- 1.7 28300 4- 300 (GrN 2092) 25800 ± 170 (GrN 1286) 29000 ± 200 (GrN 2598)
[c
17.5±1.7 d 24.8±2.3
:::: :-oG::i i-i.i.i.ii.i.i-i.i.i. '.ii-i,i31.iiii
>34000 (GrN 2105)
i.i.i.i.ii.i.ii,i,i, i.i.i-i.i-71.ii.i.i. i.i.i.i.ii-i.ii.iiDatum 2
52.9 4- 4.2
.:.!.:.!.:.!.:.!.:.!:::i:::!:::!::.!.:.~...!:
ii.i.i/.i.i.i.i.i-n, i i
Soil complex I[ J
I
!iiiiiiiiiii!ii:iiiiiiiiiii?iiiiiiiiii m,:.: j
52.4 ± 4.0 ~p51.1 ± 1.4 49.7 ± 4.5 J
>51800 (GrN 2599) 42000 ± 800 (GrN 2614) 49900 ± 600 (GrN 2152)
65.0 4- 5.0 58.0 ± 6.4 62.2 ± 5.3
::::::::::::::::::::::::::::::::::::: "~ 63.7 ± 1.5 65.2 ± 5.1 J 65.8 4- 5.1 m
78.6 ± 5.4 >50000 (GrN 2604)
iiii!i:i!ilili!i!i!i!iiiiiiiii!iii!iiiii ~ 77.4 4. 6.3 ~'- 74.2 4. 3.3 t
Soil complex
III f
:!:!:!:!:!:!:!:!:!:!:i:i:!:!:i:!:!:!:! - i: 70.9 4. 5.7 9 80.0 ± 6.4 84.3 4. 6.6 "~ 84.9 ± 0.6 85.4 4. 7.4 f
:: :::::::i::::!:i!i:i:)::i::i:"; i!::i::B.! b ::iii::iii::!::!::i::!!! !!:~:i::
•i,i,i.i.i i,i,i i.i. •i i i i i i i i : ; : ; : e D ;:l S 78.3 4- 6.5 ii-i. t
LEGEND ma Luminescence sample
83,9 4- 7.1
.iii.
iiiiii=
U 84.5 + 6.5
Gamma OA spectrometer reading
iii i
i
!
~
Loessicmaterial
~
Palaeosol
FIG. 1. Schematic di;.tgf~tlll ~t lhc Doltll \ CM~HllC'L' prt~lilc ~ho~mg IJl~: Ioc;lliOll o l Jtllllill¢SCcllCu ~,anlpl¢~,, ~~allllll;t ~,pk'clfonlctcr readings and
i',ubli~h,..,.I z . ~ d , , c a i l ' , ~ , l
dak"~
l hc II
a~c~. o l ' , t a m , . : d I-,~ I h ¢ f C ~ ¢ l l C r ; i t i O l l i l l ¢ l h o d
SAMPLE i.OCATiONS T w e n t y - o n e s a m p l e s xvcre c o l l e c t e d . T h e four y o u n g est w e r e from section 1 km to the s o u t h , w h e r e a pinkish h o r i z o n was well e x p o s e d by the e x c a v a t i o n of Ioc~,s for c o n s t r u c t i o n o f the n e a r b y d a m : in T a b l e 1 the d e p t h s given r e l a t i v e to d a t u m I, the base of the section, which was 25 cm b e n e a t h the base of the o c c u p a t i o n layer. F o u r t e e n s a m p l e s w e r e t a k e n from the m a i n section in the b r i c k y a r d a n d r e l a t i v e to d a t u m 2, the t o p o f B2g. T h e t h r e e r e m a i n i n g s a m p l e s wcrc t a k e n 2 m
are
; i r e ~.ho\ ~,11.
laterally from the m a i n section w h e r e tile u n d e r l y i n g loess was e x p o s e d by the r e m o v a l of talus.
EXPERIMENTAL
DETAILS
L u m i n e s c e n c e measurements were made on the 4-11 p.m fraction. B l e a c h i n g was carried out f o r 24 hr w i t h a S O L 2 solar s i m u l a t o r and discs were preheated for 16 hr at 140°C before measurement. All discs were left for at least 24 hr b e t w e e n each
413
L u n f i n e s c c n c c Dating ol C z e c h L o e s s TABLE
1. S a m p l e , d e p t h b e l o w d a t u m a n d c q u i v a l c m d o s e ( E D ) dct,,,lrmincd by d i f l e r c n t m e t h o d s for loess I r o m D o l n i V c s t o n i c e ,
Sitmple
Depth
TL
(cm)
,
, ,r,,
,,±el
+ ,:
,re,
l,,,,
Regeneration
Additive dose
103.0-+3.b 112.2-+8.fl 93.6-+5.2 139.1-+11.6
89.(1_+4.5 96.9-+3.4 86.9±4.4 --
Rcgcncration
,,
......
TL alter IRSL
IRSL
Short shmc
Additive dose
Regeneration
Additive dose
Regeneration
Additive dose
113.6_+7.4 116_+6.7 1112.8_++1.2 14(I.2_+11.9
121.1_+4 85.11-+2.8 94.4-+6.2 1t18.9_+1.(I
84.1_+5.3 811.1_+3.6 79.4_+5.1 96.5_+7.5
IIN.8-+ 3 83.8-+2.1 95.5-+3.5 82.8-+4.8
75.3+3.6 76.7_+3.0 8(I.S+_5.6 IO5.5+_25.6
92.9-+7 78.7-+ I.fl 72.fl-+3.11 ll)S.5-+3d~
267.4+_ 14.5 231.1+_11.1 25~1.6+_ 11.2 279.4_+15.2 29(I.9-+29.8 32(~.0_+ 17.7 314.4-+8.5 342.1_+18.2 372.6_+ I l. 1 398.<)+- 111.8 -. . 412.4_+16.3 458.7+_ 1(I.7 -398.7-+19.0 . .
240.2_+20.8 217.5_+4.1 274.9_+ 18.7 323.1-+24.2 242.4-+8.2 327.4_+ 19.3 276.11+6.8 366.3+20.9 378.(I_+9.5 491.5+-8.5 -. 448.3+32.8 ---.
2118.9_+ 14.5 -2o4.9_+ 15.1 198.4-+111.1 256.2+_21.3 3311.9_+8.4 . 295.4-+14.9 . -351.11_+3.3
248.8_+5.2 -215.1 _+ 111 I81.7_+5.1 198.1-+2.7 265.9_+2(1.5
---277.5±33.5
-----
Datum 1 a b c d c
93-103 48-58 25-35 l-HI Datum 2 -211-10
f
I2-2<1
g h i i k I m n o p q r s t u
411-5(I 711-8(I 87-93 113-121! 1411-15tl 1711-178 177-183 25(1-28t 2911-31111 318-325 3311-34(l 368-373 445-455 5(}5-515 555-565
269.6_+8.9 257.3_+3.4 254.0_+ 19.0 2811.1_+7.8 253.(1+28.3 434.8_+25.8 314.4-+8.5 351.0+25.4 --369.6_+8.2 434.5_+15.7 412.9-+11.2 427.7_+25.7 316.9_+ 19.(I 398.3-+23.1 377.5-+ 1(I.8
TL Thermotuminesccnce.
186.8_+4. I 2511.7-+7.11 249.9_+7.5 229.11-+4.6 182.8-+9.9 398 all-+ 111.8 3111.1_+15.2 373.4-+11.5 --377.1_+32.0 . 451.7_+20.1 397.9_+ 14.9 265.(I_+5.22 -.
224.5-+211. I 259.8+_6.6 195.1_+7.2 2(111.1_+5.9 227.3_+6.5 294.3_+ 11.9 253.0_+2(I.I 317.2-+25.(I 242.11+_2(I.7 21111.8-+13.3 2t111.7_+22.S 357.7_+ 27.b . . . 330.1_+211.9 310.7-+11.9 . . . 353.0_+S.4 426.11-+5.9 349.0+_28.2 512.1-+65.3 . . . 359.2-+4 412.5-+19.3 431. I _+ 15.3 -3 5 5 . 8 _+ 13.3 -363.9_+2,'.;.3 -. . .
324.9-+~.t~ -399.1+_8.6
I R S L Infra-red stimulated l u m i n e s c e n c c .
operation. Measurements were carried out in an automated Rista TL/OSL Reader (Botter-Jensen et al., 19911 with a Schott BG-39 filter being used for the infra-red stimulated luminescence (IRSL) and a Coming 7-59 and Chance Pilkington HA-3 combination for the thermoluminescence (TL). The integrated luminescence for 100 seconds IR exposure was used for the IRSL equivalent dose (ED) determination, and the same discs then had their TL measured. Thermoluminescence measurements were also made on a separate set of discs. The ED was also determined
by a 0.1 second IRSL measurement taken before the TL measurement (short shine data in Table 1). Both the regeneration and additive dose methods of ED determination were applied and the results are given in Table 1. Dose rates for all samples were obtained from the potassium content, as measured by atomic absorption spectrometry, and alpha counting of bulk samples which had been ground in a stainless-steel ball mill. The K:O content and ground alpha counts are given in Table 2, along with the total dose rate calculated
T A B L E 2. Radioactivity analyses, equivalent d o s e s ( E D s ) . total d m c r a t e s a n d a g e s c , d c u l a l c d l r o m t h e m Sample
Depth below datum
Alpha count
K,O content
ED
Total dose rate
(cm)
(ground)
("4,)
(G})
linG,, a ~)
0.95 0.93 11.96 1.03
,." 16 2.1~ _._0~ "~ 2.24
lU3.u_+3.<~ 112.2-+ s.
5.24_+0.13 5.19-+0.14 5.32_+11.15 5.,'Q_+I). 15
19.71111_+ 16<111 21 .NM)_+ 19(11) 17.5(1(I_+ 17(111 24..sa )fl-+ 231111
0.90 O.85 0.89 11.73 I).74 0.94 11.83 0.96 11.78 (I.89 0.911 fl.95 11.85 fl.85 (I.78 (I.78 I).64
.- _
2(lt).6+_S.9
2.211 _"~ . _ 8"~ ' 2.1)fl 2.1111 2.211 _.,fl'~ ~ _._11 "~ "~ 2.3O 2.311 2.36 2.411 2.18 '3 "3 _.3_ 2.18 2.36 2.68
257.3_+34 254.11+ 19.11 28 I. I -+7 S 253.11+_38.3 3 2 6 . f l + 17.7 314.4-+N.5 351 . I+~q __ 4 3 7 2 . 6 ± I I. 1 39.'-;.11_ +_ 1{I.8 369,6_+ 8.2 4 3 4 . 5 _+ 15.7 4 1 ~_ . ~) _+ 11._ 4 _3 7 . 7 _- 4_- 3 ~- . 7 361.t1±19.11 398.3_+23. I 3 7 7 . 5 +_ Iq).8
5. Ill-+ft. 14 4.t) l _+0.13 5.11 ±ft. 14 4.31 -+I). 12 4.3(i-+11.12 5.24-+11.14 4.82+o_. 13 5.33_+11.15 4.74_+11.13 5.14_+11.14 5.21 ±ft. 14 5.43_+0.15 4,9()+(I.13 . q.l}l _+(I. 13 4.6__0.~+ 1_'~ 4.75_+O. 1 I 4.47-+O. 12
52.9(11)_+ 42(111 52,4111)_+4111111 49.7(M)_+ 45(111 fi5 ,(1(11I-+ 5(11111 58.(11111_+64(111 62,2(111-+53(111 /+5,21R)-+5 I<111 fi5,bl[111-+51111) 78.6(111-+64(111 77.41R)+ 631111 711.9(11)_+571R)
Age ()'cars)
( c t / u n t s ks I c m 2)
Datum 1 a b c d e f g
h i j k
1 m n o p q
r s t u
93-1113 48-58 25-35 l-Ifl Datum 2 -2(1-111 2-2fl 411-511 71>811 87-93 113-1211 14(1-15fl 171#-178 177-183 251#-2611 291t-311(I 318-325 330-340 368-373 44q~tSq. .. 5(15-515 555-565
£0,(1111)-+ 64(R) 84.300+_ (~(lll 85,400-+ 74(•) 78,3(10-+65011 83,91111-+7 I1111 84,5(111_+1"15(111
F.M. M u s s o n and A . G . Wimlc
414
"IAI?,LE 3. A l p h a count rates and dr',', inlinitc beta (lose rates calculated lrom thick ~,ourcc alpha (TSA(') and beta (TSBC) counting Sanlplc
D e p t h bclm~ dattun (cm)
Alpha count crushed
A l p h a count ground
(countslkscc/cm 2)
(counls/kscc/cm 2)
A l p h a count ratio (crush/groundl
(I.92 1.24 I. 12 1.24
I).95 11.93 ILt)6 1.03
11.97 . ~, I.'~ I. 17 1,20
I.()S 1. I3 t).93 0.S6 ().S4 I.(ll I. 16 0.gN
O.tlU ().8g 0.89 11.73 0.74 11.t)4 (I.83 (I.t)(~ 7,',;
1.20 1 ~'~ 1.0S I . IS 1.14 I.(17 1.411 I.t12
I)atum I 93-103 4S-5S 25-35 1 Ill I)al tllll 2 - 2 0 - 111 I--40-51t 71l-gll S7 93 113-1211 1411 150 I70-17S 177- IS3
a b
c d c 1 ,, h i i k 1 m II
_
o
.
- -
.
r s t u
--
(I.,,49
I).92 1.18
0.90 0.95
(G_6(I
290-300 3IX 325
p
.
3aS-373 44~,-4~ . ~ 5(15-515 555-565
If.
.
.
I. IIt) {I.g6 (I.89 1.113
.
.
11.85 0.78 . ().7s I).h4
"I'ABI.E 4. C o m p a r i s o n ol g a m m a a n d beta (lose late', { i n ( i \ a i) c a l c u l a t e d J'lOlll different lllCiiMilClllClll,,
A B C D IZ F G
A
1.34_+11.(~4 1.37+OUl 1.37_+(L03 1.25+_0.02 1.411_+1L113 1.35+11.(13 1.23_+o.03 '
B:
(::
I )~
.40=cuC
2.25+_H{pu 2.17+{L12
42+_o.o2
2.37_+u.1~4 2.41 _+(I.It4 2.13_*.1UI4 2.52+1L1~4 2.33+11.04
.51 *_11.1)2 .411+_1L112 .3II_+(LII2 .222[UI2 .411+_0.02
2.211"_1L(14
2.27+(i.1ff~ 2.3N_+11.05 2.4S_+0. Ill 2.5h~0.11 2.~7~11 tl4 2.4114-_11.115
hi 3#11 gallllll~lusing gi.lllllllilspccIrOlllCt,2r CldlllCl1[ COlllpOMtion.
-, I) D g a m m a c a h : u l a l c d using g r o u n d a l p h a count and K,(). :~: Beta dose rates using galllllla spectrometer ¢lclllCnI composition tlS,Jllg ill Sitl¢ water contellt, Bctll dr)so raD2s fronl dr~ beta couiitiil~.
Beta dose rate TSB(" ( m G 3 a I)
_._9( "~ "~ _..~81 "~. - ' 2.583 2.635
2.318 " "~114 2.437 2.448
2.33-+O. lO 2.35_+0.1H 2.51 _+IL04 2,43_+t1.05
2.3114
2.41 +0.O4 2.40_+0.02 _.4)_IL0_'~ -+ "~ 2. I l +11.[11 2.10-+IL06 2.34_+ 0.1if-, 2.4()_+1).II2 2.40_+11.03
.
1.2S 1.. Ill 1.14 1.60
.
3 _.3_9 2.510 "~ "~S'~ 2. 134 2.114 2.4O3 2.53(~ _.37 . .
- -
assuming a water content of 20% based on natural water contents measured on Czech loess (Feda. 19661 and laboratory measurements. The measured in situ water content at 30 cm from the surface was only 5% as the site was visited in the summer. Alpha counting of samples crushed with a pestle and mortar (Table 3) gave counts which were consistently higher than the ground alpha counts (from 1.05 to 1.60 times) with one exception (sample a). These results contirm the study of Z611er and Pernicka (1989), who found cvidence of over-counting for coarser material. Use of the ground sample alpha counts in the dose rate calculations gave ages in better stratigraphic ordcr. Thick source beta counting (Sanderson. 1988) was also employed using 15 g crushed samples, which ~vcre counted fl~r three hours in rotation with MgO (background)and a standard composed of powdcrcd Shap Granitc (with a dry beta dose rate o f 6.25 m G v a t). T h c dry bcta d o s e rates obtained in this wa~ arc given in Table 3. It was not possible to m a k c g a m n l a s p e c t r o m e t e r readings down the whole section. However. ,(t s e v e n
Sample
Beta dose rate ground TSA(" I m O y a ~)
.
_.4.~ " "I 2.691
1.02 1.24
.
.
.
Beta dose rate crushed TSA(" ( m G y a ~)
2.349 2.014 .~.0_. " '", 2.33~ 2,336 .
.
. _._70 2.36(t 2.411 2.484 4~
2.73_+11.117 _.44_0.06 _. 14+0.04
_.:,44 "~ "
2.66_+(I.I15
_." 179 2.3113 2.40()
~ ~(1+_(1.1)7 2.17_+( I14 2.32_+0.115
.
2.55 ~ . ."~. . "~.~t) . 2.409 2.748
- -
locations (shown in Fig. 1) the gamma dose rate was recorded in situ (Table 4). For samples from these locations the gamma dose rate was also calculated from the ground alpha count and K~O analysis. Also in Table 4, the values of the beta dose rates calculated from the four-channel gamma spectrometer readings are compared with those from thick source beta counting (TSBC). The in situ water content was < 5 %.
DISCUSSION Equivalent doses were obtained by both the regeneration and additive dose TL methods (by integrating the 25(I--401)°C region of the glow' curve) on discs which had previously had their IRSL signal measured for I()0 seconds. The E D s for the regeneration method w e r e on average 15% higher, suggesting that no sensitivity increase has resulted from the laboratory bleach. For ,wmn"er~ samples, which give TL age estimates of 6() ka or less, there appears to have been a sensitivity decrease as a result of laboratory bleaching. For the set of discs on which TL alone was measured, only I. o a n d q show higher E D s for the additive dose method. Although these three are towards the base of the section, this effect does not appear to bc systematic, as samples r and s show the inverse. For the second set of TL measurements (those after the 1()0 sec IR exposure), the EI)s for lhc additive dose method tended to be higher for the older samples, with i and k being exceptions. The values of ED by the regeneration method for TL are shown in Fig. 2. The E D s for samples s. t and u arc lower than for the overlying samples as a result of their lower total dose rates (Table 2). These are the values used to obtain the
415
l.ummescencc Dating of Czech Loess o 2 ,t a.
[;
-C
t
-2 1o
•
A&
1:I1
I)
1O0
:20o
300
-I00
500
El) ((;v FIG.
_"
Equiv,dcnt
dose vs. depth curve measurements.
for
regeneration
ages in Table 2. These EDs were selected as there was a more complete data set and the results were much less scattered on a depth-dose plot (Fig. 2). (Note that the EDs are from the first set of T L measurements, apart from those for samples m. n and j.) For the IRSL measurements the regeneration/additive dose ratio is 0.91 for the 100 sec IR exposure and 1.02 for the short shine. However, the IRSL EDs arc smaller than those for the TL; for the regeneration data sets obtained on the same discs, the mean IRSL underestimate was 15%. This may be due to the incomplete removal of the unstable part of the IRSL signal by the preheat. The 16 hr preheat was selected following storage experiments at 140°C. After 10 hr at this temperature the IRSL signal from natural and irradiated aliquots attained a fixed ratio. The short shine regeneration EDs are slightly lower still, being on average 5 % lower than those for the signal integrated over the first 1{)(Isec. This may indicate a loss of natural signal as a result of laboratory illumination during the disc preparation. For the following discussion the ages are those given in Table 2 based on the ED obtained using TL regeneration measurements. Five groups can be recogniscd which are related to the visible soil horizons and the loess overlying the pinkish layer. The mean T L ages arc ,aiven in Table 4 and may be compared with other age evidence. The TL ages relating to the loess overlying the pinkish laver (a, b and c) are in agrcemcnt with the radiocarbon ages. The loess beneath the pinkish layer (d) appears to be younger than expected on the basis of the radiocarbon dates for the occupation laver at the excavation site, and the quoted uncalibrated radiocarbon dates are already about 2 ka less than the calendar age (Bard et al., 1990). The most likch explanation for such a discrepancy is that the pinkish horizon at the sampling site is rcworkcd matcrial derived from a nearby occupation horizon, althou,,he there is no sedimentological evidence to support this hypothesis. No such age discrepancy was encountered when dating loess at Stillfried, Austria, from a horizon with a similar radiocarbon age (Wintle, 1987). The ages of the older groups are systematically
younger than might be expected according to the interpretations of Kukla and Koci (1972) and Sibrava (1979). The chernozems B2g and B2b do not fall within the accepted age range of stage 5, 129.7-73.9 ka (Martinson et al.. 1987) and soil complex Ill ( B i b and Bid) does not fall within the range for substage 5e, 129.7-122.5 ka. Using the same TL dating procedures, loess bracketing the substage 5e soil in Hungary gave ages of about 76 ka (Wintle and Packman, 1988) and in Yugoslavia ages of 7 5 - 8 5 ka (Singhvi et al., 1989). Similar age limitation has been reported elsewhere in Europe (see Wintle, 1990, for further discussion), but not in New Zealand (Berger et al., 1992). A possible cause of underestimation, particularly if there is no obvious sensitivity change (as evidenced by the lack of systematic underestimation of the regeneration ED compared with that obtained by the additive dose method) is the long-term loss of luminescence centres (Debenham, 1985). If this loss can be characterised by a formula of the form T = "r(l-e '/') where -r is the lifetime of the decay and t is the true geological age, then the luminescence age T can be predicted if a suitable value of "r is proposed. "r is the limiting age which would be obtained for an infinitely old sample which behaves in this manner, and a value close to 150 ka has been suggested (Wintle, 1990). Taking the Oxygen Isotope Stage 4/3 boundary as 59 ka, and the end of the Oxygen Isotope Substages 5a, 5c and 5e as 73.9, 96.2 and 122.5 ka, respectively (Martinson et al., 1987), the corresponding luminescence ages calculated from the above formula would be: 4/3 boundary, 7" = 48.8 ka: 5a/4 T = 58.4 ka; 5c/5b T = 71.0 ka: and 5e/5d T = 83.7 ka. Assuming that the average luminescence ages of a soil will be biased toward the end of the warm phase giving rise to it, the TL ages for groups 2, 3, 4 and 5 in Table 5 can be compared with the ages calculated above. (The assumption is based on the role of bioturbation in bringing mineral grains to the surface where they arc exposed to light.) It should be pointed out that this model only takes into account a simple time-dependent behaviour. This may need to be modified, ahmg the lines suggested by Mejdahl (1988), to incorporate the additional effect of variable dose rates from site to site. This is not easily demonstrated for European loess, for which most dose
T A B L E 5. Mean lhcrmoluminesccnec (TL) ages for lhc main straligraphic units at the Dolni Veslonice section Group 1 2 3 4 5
Unils
M e a n T L a g e (ka)
Samples
Pinkish laver and loess above Upper chcrnozem B2g Lower chernozem B2h ('hernozcm Bid Para-brown earth B I b
19.6+ 1.7
a,b,e
51.1_+1.4 63.7+ 1.5 74.2_+3.3 84.9+0.6
f,g i,k n.o q.r
416
F.M. Musson and A.G. Wintlc
rates arc of the order of 4 to 6 m G y a -~. as at Dolni Vestonice. Ages well over the 150 ka limit proposed bv D e b e n h a m (1985) have been obtained in New Zealand (Berger et al., 1992), where much lower dose rates were measured. Occasionally, older ages are also found in Europe where the dose rates are lower. In spite of this limitation, the effect of D e b e n h a m ' s model at this site (where dose rates range from 4.31 to 5.62 m G y a -~) was explored in terms of the ages proposed by others (Kukla and Kocf, 1972; Sibrava, 1979) for the two soil complexes II and II1. Comparison of the effective TL ages calculated for the oxygen isotope boundaries (given above), assuming the model with the mean TL ages obtained for the observable soil units (Table 5), suggests that the soils are in any case wrongly grouped. The para-brown earth ( B l b ) is of a different age from the overlying chernozem (Bld). According to the model proposed in this paper, the para-brown earth ( B i b ) would correspond to substage 5e, the overlying chernozem (Bld) to substage 5c, and the lower chernozem (B2b) of soil complex lI to substage 5a. This suggests that a much higher rate of loess deposition occurred in this region during substage 5b than 5d. The upper chernozem (B2g) would have formed significantly later, although it seems too old to relate to the warm phase at the beginning of stage 3 (event 3.31 of Martinson et al., 1987), which has an astronomical age of 55 ka, and thus would give a calculated T L age of 43 ka. Instead, a warm phase in stage 4 ending around 63 ka would need to be invoked to result in the T L age of 51 ka obtained for group 2. However, it should be noted that the age of the loess 15 cm above the top of B2g (sample e) has a similar age; B2g could therefore represent reworked humic material from an earlier chernozem, e.g. B2b. Studies of soil micromorphology are required to investigate this hypothesis.
CONCLUSIONS The TL age estimates obtained for samples from the four soils (para-brown earth and three chernozems), which are superimposed at Dolnf Vestonice, suggest that they formed at different times within the last interglacial-glacial cycle, in contradiction to previous interpretations based on sedimentological analyses. H o w e v e r , the age estimates from the TL and IRSL m e a s u r e m e n t s using regeneration procedures underestimate the ages, as has been found for other sites in central Europe. The interpretation explored in this paper, based on D e b e n h a m ' s hypothesis of luminescence centre decay with a lifetime of 15{) ka, is that the para-brown earth, and the two overlying chernozems at Dolnf Vestonice, relate to Oxygen Isotope Substages 5e, 5c and 5a, respectively. Comparison of the E D s from TL and I R S L m e a s u r e m e n t s on the same discs suggest that a 16 hr preheat at 140°C is not adequate to isolate a stable I R S L signal. The behaviour giving rise to
the age underestimation in T L also affects the IRSL measurements.
ACKNOWLEDGEMENTS The authors thank Dr M.L. Clarke for her critical comments and assistance with the preparation of the manuscript, and H. Edwards for determination of the K~O contents. AGW thanks Dr R. Grtin and Dr P. Havlicek of the Czech Geological Survey for assistance in the field and Professor H.P. Schwarcz for the loan of the gamma spectrometer. Financial assistance was provided by NERC grant GR3/7242. This is publication number 349 of the Institute of Earth Studies, University of Wales, Aberystwyth.
REFERENCES Bard, E., Hamelin, B., Fairbanks, R.G. and Zindler, A. (1990). Calibration of the lzC timescale over the last 30,000 years using mass spectrometric U-Th ages from Barbados corals. Nature, 345, 405-41(I. Berger, G.W., Pillans, B.J. and Palmer, A.S. (1992). Dating loess up to 800 ka by thermoluminescence. Geology, 20, 403-406. Botter-Jensen, L., Ditlefsen, C. and Mejdahl, V. (19911. Combined OSL (infrared) and TL studies of feldspars. Nuclear Tracks and Radiation Measurements, 18, 379-384. Debenham, N.C. (1985). Use of U.V. emissions in TL dating of sediments. Nuclear Tracks and Radiation Measurements, 10, 717-724. Feda, J. (1966). Structural stability of subsident loess soil from Praha-Dejvice. Engineering Geology, !, 2[11-219. Kukla, J. (1970). Correlation between Ioesses and deep sea sediments. Geologiska F6reningen i Stockhohn FOrhandingar, 92, 148-1811. Kukla, G.J. and Koci, A. (1972). End of the last interglacial in the loess record. Quaterna O' Research. 2, 374-383. Mejdahl, V. (1988). Long term stability of the TL signal in alkali feldspars. Quaterna O' Science Reviews, 7, 357-36[). Martinson, D.G., Pisias, N.G.. Hays, J.D., Imbrie, J., Moore, T.C., Jr and Shackleton, N.J. (19871. Age dating and the orbital theory of the Ice Ages: development of a high-resolution 0 to 300,00[~-year chronostratigraphy. Quaternary Research. 27, 1-29. Sanderson, D.C.W. (1988). Thick source beta counting (TSBC): A rapid method for measuring beta doserates. Nuclear Tracks and Radiation Measurements, 14, 2113-2(/7. Sibrava, V. (1979) ht: Guide to Excursions ~1 the Sixth Session of the IGCP Project 24, pp. 65-6,6. Geological Survey, Prague. Singhvi, A.K., Brongcr, A.. Saucr, W. and Pant, R.K. (1989). Thermoluminescence dating of Ioess-palaeosol sequences in the Carpathian Basin (cast-central Europe): A suggestion for a revised chronology. (Jwmical Geology, 73, 307-317. Vogel, J.C. and Zagwijn. W.H. (1967). Groningen radiocarbon dates, VI. Radiocarbon. 9, 63-1116. Wintle, A.G. (1987). Thermoluminescence dating of loess at Rocourt, Belgium. Geologic en Minjhouw, 66, 35-42. Wintle, A.G. (1990). A review of current research on TL dating of loess. Quaternary' Science Reviews, 9, 385-398. Wintle, A.G. and Packman, S.C. (19881. Thermoluminescence ages for three sections in Hungary. Quaternary Science Reviews, 7, 315-320. Z611er, L. and Pernieka, E. (19891. A note on overcounting in alpha counters and its elimination. Ancient TL, 7, 11-13.